This disclosure is concerned with a specific combination of a deep tank fermentation system and a flotation system for initial separation of the resulting solid material and liquid materials. Contrary to most flotation systems, this arrangement requires no external air supply to operate the flotation apparatus. The required gaseous bubbles are created by depressurizing the liquid in the resulting suspension that is produced in the deep tank reactor.
The feasibility of deep tank reactors for fermentation processes is generally described in U.S. Pat. No. 3,574,331, granted to Kurosawa on Apr. 13, 1971. In general, the use of a deep tank for treatment of a slurry of sewage or industrial waste is described in that patent. The slurry absorbs oxygen and is strongly agitated by air discharged into the tank. The use of a deep tank and resulting increased hydrostatic pressures increases the rate of oxygen transfer in the biological process. The patent specially mentions that the high concentration of available oxygen in the waste aerated at a high pressure reduces the dwell time of the waste in the tank to produce adequate activation of the sludge when compared to conventional, relatively shallow aeration tanks where the hydrostatic pressure at the aeration nozzles is not normally higher than 5 p.s.i.g. In the patented disclosure, the sludge is removed from the top of the deep tank and cascades downwardly to a gravity settling tank.
In the present disclosure, the completely mixed contents from a deep tank are directed to a flotation tank, where the liquid pressure is suddenly decreased. This sudden depressurization of the liquid, which is then momentarily supersaturated with gas, causes the gas to form bubbles which in turn are used for flotation purposes. It is the combination of the deep tank fermentation system and the flotation system for removal of sludge which constitutes the basis for this disclosure.
The prior U.S. Pat. No. 3,542,675, which was granted Nov. 24, 1970, shows a pressurized tank for liquid that is fed to the inlet of a flotation tank and mixed with waste water in the flotation tank to thereby separate the solid contaminants from the desired water. It is not directly related to a deep tank fermentation system.
U.S. Pat. Nos. 3,576,738 to Duffy, issued Apr. 27, 1971 and U.S. Pat. No. 3,725,264 to Wheeler issued Apr. 3, 1973 show waste treatment processes where the processing of the waste water occurs in a pressurized system, the resulting liquid suspension being directed to a flotation cell through a back pressure valve. The flotation cell is maintained at atmospheric pressure and as the waste water which is saturated with gas under pressure enters the cell, the reduction in pressure causes the gas to be released from solution in the form of small bubbles which attach to oil particles, residual solids, etc. and carry them to the surface of the water in the flotation cell.
The present disclosure is unique in its combination of a deep reactor tank for fermentation, the tank being open to the atmosphere and relatively simple in both structure and operation. I have found it to be practical to use the resulting dissolved gases in the liquid as a source of gaseous bubbles for flotation separation processes.